But the Oklahoma Republican's Senate bid has picked up momentum ahead of Tuesday's primary, which now looks likely proceed to a runoff, giving Lankford his clearest shot at the open seat.

To win the GOP nod, the two-term congressman must eclipse the national star power of his most formidable opponent, former state Speaker T.W. Shannon, in a battle that also includes former state Sen. Randy Brogdon and several lesser-known candidates. If no candidate receives 50 percent of the primary vote Tuesday, the top two vote recipients head to an August runoff.

Lankford's recent rise in polls and on the airwaves are signals that scenario, and his chances of taking the nomination, are increasingly likely, according to Sooner State Republicans.

“It seems to me now like Lankford has the momentum,” said Oklahoma Republican Party Chairman Dave Weston.

That’s a shift from six weeks ago, when Weston said momentum was on Shannon’s side. After all, national Republicans had zeroed in on the Senate hopeful with all the makings of a future GOP superstar: an enrolled member of the Chickasaw Nation, the first black state speaker and the youngest person to ever hold the position.

Shannon went on the airwaves early in the race, building his familiarity with voters statewide. Sens. Mike Lee, R-Utah, and Ted Cruz, R-Texas, endorsed Shannon — he was one of only two Senate candidates Cruz has publicly backed this cycle — and along with Sarah Palin, they rallied for him in April.

But that was before Lankford started his own media campaign in earnest, building on his existing statewide presence grounded in the state's burgeoning Baptist community. Mike McCarville, who runs the Oklahoma political blog the McCarville Report, agreed that in the final week of the primary, it is now "advantage Lankford."

Now polling shows Lankford moving ahead , although all recent public surveys on the race have been sponsored by the campaigns or super PACs backing them. An early June internal poll from Lankford's campaign showed him leading Shannon, 41 percent to 34 percent. That's a slightly narrower lead than an internal poll from a month before, when Lankford led 43 percent to 33 percent.

The most recent surveys from Shannon's supporters put him narrowly ahead of Lankford, 39 percent to 37 percent. But there's a small shift from April, when a similar poll from the same pro-Shannon super PAC showed him ahead of Lankford, 42 percent to 32 percent.

Lankford's pitch is that while Shannon might look and sound good, the congressman is a workhorse who will actually get things done.

“He sees himself as a messenger, he likes to carry a national message, he likes to carry that out,” Lankford said in a June 10 interview with CQ Roll Call. “I see myself as more of a student, that I love to get down into the weeds of different problems and try to go through that. I don’t mind messaging, but I’m going to default back to the research side of things.”

In a May debate, Lankford lamented, “the reality of going into Washington, D.C., and running into a lot of folks that are there because they like to be celebrities. They like to get on TV. They like to make statements. ... You’d ask them hard questions after they’d made a television statement and they didn’t really know the issues. They just knew how to do talking points.”

In a June 13 interview with CQ Roll Call, Shannon portrayed himself as strong messenger for his party with a track record of accomplishments.

"It’s not enough just to be right anymore, we also have to elect leaders who can go out and convince other people that we’re right," he said, adding that this is why the party has not "been as successful nationally" in recent years.

"I’ve got a record of not just talking about conservative ideas, not just talking about how great it would be, but actually moving conservative ideas into law," continued Shannon, who never said Lankford's name in a 10-minute phone interview.

Despite Lankford’s momentum, Weston sees about a 70 percent chance of the race continuing to an Aug. 26 runoff.

Lankford joked that his wife, tired of living with a man on the campaign trail, had forbidden that word to be spoken in their house.

But in a runoff with Shannon, Lankford has a built-in advantage: His home turf. A crowded race for his House district is also on track for a runoff, which means there will be multiple voter turnout operations in Lankford's geographical base on the same date.

As a result, Lankford is likely to easily win Oklahoma City, the biggest media market in the state. Shannon hasfocused his efforts on Tulsa , the second largest media market, in an effort to counter that advantage.

On Tuesday, Brogdon voters will make the difference between Republicans picking a nominee and two more months of runoff campaigning. He's received between 4 percent and 6 percent in recent polls — enough to keep Shannon and Lankford below the necessary 50 percent threshold in a tight race — but he got 40 percent of the vote in the 2010 gubernatorial primary, making him something of a wild card.

But it's unclear which candidate would pick up Brogdon's supporters in a runoff. Several local tea party groups backing Brogdon have been vocal in their dislike for Shannon, calling him not conservative enough in a public letter to national groups.

That should signal they would support Lankford in a runoff, but it's not evident they like the congressman any better. As a result, there's a big possibility this group of Republicans could stay home in late August.

This is the special election to succeed Sen. Tom Coburn, R-Okla., who is resigning at the end of this Congress. The winner of the primary will almost certainly be the next Senator from this solidly Republican state.

The race is rated Safe Republican by the Rothenberg Political Report/Roll Call.

Correction, June 18, 2:18 p.m. An earlier version of this post incorrectly reported when Brogdon ran for governor.